[campaign #239] STEP 4-A — STDLIB FILL II: raw binary I/O (Bytes constructor + LE getters/setters) #326

Part of #239, follow-up to #242 (STEP 3) and pre-req for #243 (STEP 4 per-repo ports).

Why this is needed

The STEP 4 triage on #243 (2026-05-31) found that every estate test repo carrying a binary-layout ABI contract — raze-tui (6 files, 1643L), eventual tree-sitter-k9 / tree-sitter-a2ml ports, half of bofig — builds its test fixtures with:

const buf = new ArrayBuffer(16);
const view = new DataView(buf);
view.setInt32(0, kind, true);        // LE
view.setUint16(10, mouse_x, true);   // LE
view.setUint8(8, modifiers);
return new Uint8Array(buf);

and parses with the symmetric getInt32(0, true) / getUint16(10, true) / getUint8(8).

#242 / affinescript#504 added the read-only accessors that pure-logic ports of the STEP-2 set needed (bytesLength, bytesByteAt, bytesAsciiSlice). They do NOT cover Bytes construction, per-field write at offset, or multi-byte LE read — which is what binary-ABI tests need.

Required surface

All little-endian (the estate C ABIs are LE-pinned per raze-tui's raze-events.ads + Idris2 Events.idr):

Implementation pattern: same as #504 — extern declarations in stdlib/Deno.affine, lowering table entries in lib/codegen_deno.ml's deno_builtins, smoke test under tests/codegen-deno/.

Out of scope for this issue:

• Big-endian variants (the estate doesn't need them today; revisit if/when an external API forces BE).
• 64-bit getters/setters (getBigInt64 / setBigInt64) — defer to a *_i64_le follow-up if needed; current STEP 4 candidates max out at 32-bit.

Acceptance

• All 10 externs ship in stdlib/Deno.affine with docstrings.
• tests/codegen-deno/bytes_binary_io.{affine,deno.js,harness.mjs} round-trips a 16-byte raze-tui-shaped record (LE i32 + u32 + u8 + u16 × 2 with explicit pad bytes), with build + parse asserting field-level equality.
• dune runtest green; tools/run_codegen_deno_tests.sh green.

Refs

• Umbrella: [campaign] TypeScript → AffineScript estate migration (UMBRELLA) #239
• Predecessor: [campaign #239] STEP 3 — STDLIB FILL: discharge gaps surfaced by step 2 #242 (STEP 3) + affinescript#504 (the read-only-Bytes slice)
• Direct unblock: [campaign #239] STEP 4 — TAIL BATCH 2: mid-tier pure-logic repos (4-9 files) #243 (STEP 4) — specifically raze-tui's 6 files